The present invention relates to an apparatus for stabilizing the intensity of light from a light source. More particularly, the present invention relates to an intensity stabilizing apparatus that is suitable for use with a light source which is incapable of direct modulation of light intensity.
The light emitted by a laser has a number of superior properties such as a high degree of collimation, high intensity and narrow spectral width. Because of these properties, lasers have been extensively used in physicochemical experiments and in various industrial fields. However, in such laser applications as photometric calibration and high-precision measurements, fluctuations in the intensity of laser light can be a problem. Sources of such fluctuations are versatile; they may be variations in the power supply, changes in the length of resonator due to thermal causes, or variations caused by contention between modes. In gas lasers, plasma noise and unstable excitation of light are potential causes of fluctuations. In water-cooled lasers, microphonic noise which is generated by resonance in the flow of cooling water may cause intensity fluctuations. Dye lasers are highly susceptible to spike oscillation that may be caused by the formation of air bubbles in a dye solution or by inhomogeneity in dye concentration and the resulting variation in the intensity of laser light is usually no less than 10%.
If the light source with which stabilization in the intensity of light is required is a laser diode or some other device that is capable of direct modulation of light intensity, the intensity of light to be emitted can be converged to a target value by applying negative feedback to the drive unit for the light source, with part of the light from the light source being monitored. However, with other sources of laser light, the intensity of light to be emitted cannot be directly modulated and a separate means must be provided for stabilizing the intensity of emitted light.
FIG. 13 is a block diagram of an apparatus conventionally used to stabilize the intensity of light emitted from a light source. In this apparatus, light emanating from a light source 10 is passed through a polarizer 12 to extract light having a single component of the polarization plane. The extracted light is then launched into an optical modulator 14 such as a Pockels cell that is capable of controlling the state of polarization by means of an applied electrical signal. The light coming out of the optical modulator 14 is passed through an analyzer 16 and thence launched into a beam splitting means 18. The analyzer 16 detects light having a component of the polarization plane that crosses at a certain angle, typically at 90 degrees, with the component that is transmitted by the polarizer 12. The beam splitting means 18 divides the incident light into two light beams, one of which will be immediately produced as an output. The other is fed into a light intensity detector 20 and its intensity is detected. After being amplified in an amplifier 22 when necessitated, the light from the detector 20 is converted into a negative feedback signal in a control signal generator 24 and the signal is then applied to the optical modulator 14. The purpose of applying the negative feedback signal to the optical modulator 14 is to ensure that when the intensity of light is increased by .DELTA.i, the optical modulator 14 will rotate the polarization plane in such a direction as to reduce the intensity of light passing through the analyzer 16 by an amount corresponding to .DELTA.i.
However, this conventional apparatus for stabilizing the intensity of light has had the following problems. First, the need to use the polarizer 12 and analyzer 16 renders the optical system unduly complicated and increases the number of parts or components of the apparatus. Secondly, the optical modulator 14 has no amplifying capability and is solely intended to control light transmittance. Thus, as shown in FIG. 14, the output light from the apparatus becomes low in intensity and the maximum intensity of output light that can be obtained by stabilization is determined by the minimum intensity of laser light that is being subjected to fluctuations in intensity.